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><A
NAME="AEN1762"
>7.2. Creating a swap space</A
></H1
><P
>A swap file is an ordinary file; it is in no way special
	to the kernel.	The only thing that matters to the kernel is that it
	has no holes, and that it is prepared for use with
	<B
CLASS="COMMAND"
>mkswap</B
>.  It must reside on a local disk, however;
	it can't reside in a filesystem that has been mounted
	over NFS due to implementation reasons.</P
><P
>The bit about holes is important. The swap file reserves
	the disk space so that the kernel can quickly swap out a page
	without having to go through all the things that are necessary
	when allocating a disk sector to a file.  The kernel merely
	uses any sectors that have already been allocated to the file.
	Because a hole in a file means that there are no disk sectors
	allocated (for that place in the file), it is not good for the
	kernel to try to use them.</P
><P
>One good way to create the swap file without holes is through
	the following command:

        <TABLE
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><TR
><TD
><FONT
COLOR="#000000"
><PRE
CLASS="SCREEN"
>	<TT
CLASS="PROMPT"
>$</TT
> <TT
CLASS="USERINPUT"
><B
>dd if=/dev/zero of=/extra-swap bs=1024 
	count=1024</B
></TT
>
	<TT
CLASS="COMPUTEROUTPUT"
>1024+0 records in
	1024+0 records out</TT
>
	<TT
CLASS="PROMPT"
>$</TT
>
	</PRE
></FONT
></TD
></TR
></TABLE
>

	where <TT
CLASS="FILENAME"
>/extra-swap</TT
> is the name of the swap
	file and the size of is given after the <TT
CLASS="LITERAL"
>count=</TT
>.
	It is best for the size to be a multiple of 4, because the
	kernel writes out <I
CLASS="GLOSSTERM"
>memory pages</I
>, which
	are 4 kilobytes in size.  If the size is not a multiple of 4,
	the last couple of kilobytes may be unused.</P
><P
>A swap partition is also not special in any way.	You create
	it just like any other partition; the only difference is that
	it is used as a raw partition, that is, it will not contain any
	filesystem at all.  It is a good idea to mark swap partitions
	as type 82 (Linux swap); this will the make partition listings
	clearer, even though it is not strictly necessary to the
	kernel.</P
><P
>After you have created a swap file or a swap partition, you
	need to write a signature to its beginning; this contains some
	administrative information and is used by the kernel.  The
	command to do this is <B
CLASS="COMMAND"
>mkswap</B
>, used like this:

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><FONT
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><PRE
CLASS="SCREEN"
>	<TT
CLASS="PROMPT"
>$</TT
> <TT
CLASS="USERINPUT"
><B
>mkswap /extra-swap 1024</B
></TT
>
	<TT
CLASS="COMPUTEROUTPUT"
>Setting up swapspace, size = 1044480 
	bytes</TT
>
	<TT
CLASS="PROMPT"
>$</TT
>
	</PRE
></FONT
></TD
></TR
></TABLE
>

	Note that the swap space is still not in use yet: it exists,
	but the kernel does not use it to provide virtual memory.</P
><P
>You should be very careful when using
	<B
CLASS="COMMAND"
>mkswap</B
>, since it does not check that the
	file or partition isn't used for anything else.  <EM
>You
	can easily overwrite important files and partitions with
	<B
CLASS="COMMAND"
>mkswap</B
>!</EM
> Fortunately, you should
	only need to use <B
CLASS="COMMAND"
>mkswap</B
> when you install
	your system.</P
><P
>The Linux memory manager limits the size of each swap space to
	about 127 MB (for various technical reasons, the actual limit
	is (4096-10) * 8 * 4096 = 133890048$ bytes, or
	127.6875 megabytes).  You can, however, use up to
	8 swap spaces simultaneously, for a total of almost
	1 GB.

		<A
NAME="AEN1790"
HREF="#FTN.AEN1790"
>[1]</A
>
	</P
><P
>This is actually no longer true, this section is slated
	for a rewrite Real Soon Now (tm).  With newer kernels and versions
	of the <B
CLASS="COMMAND"
>mkswap</B
> command the actual limit depends on
	architecture.  For i386 and compatibles it is 2Gigabytes, other
	architectures vary.  Consult the mkswap(8) manual page for more
	details.</P
></DIV
><H3
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>Notes</H3
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><A
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HREF="x1762.html#AEN1790"
>[1]</A
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><P
>A gigabyte here, a gigabyte there, pretty
		soon we start talking about real memory.</P
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